DC/DC converter, control circuit and control method thereof, power supply, power adapter and electronic apparatus using the same

ABSTRACT

A control circuit of a DC/DC converter includes: a pulse modulator configured to generate a pulse signal; and a driver configured to switch a switching transistor based on the pulse signal. The pulse modulator includes an on signal generator to generate an on signal. The on signal generator includes: a bottom detection comparator configured to compare a voltage of one end of an auxiliary winding with a predetermined threshold voltage and generate a bottom detection signal; a first time-out circuit configured to generate a first time-out signal asserted when the bottom detection signal is not asserted; a second time-out circuit configured to generate a second time-out signal asserted when the bottom detection signal is not asserted; and a logic part configured to generate the on signal based on the bottom detection signal, the first time-out signal and the second time-out signal.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2012-199641, filed on Sep. 11, 2012, theentire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to a DC/DC converter, and a controlcircuit and control method thereof.

BACKGROUND

Appliances including televisions, refrigerators and so on are operatedwith external commercial AC (Alternating Current) power. Electronicapparatuses including laptop computers, mobile terminals, PDAs (PersonalDigital Assistants) and so on are also operated with commercial AC powerand their internal batteries may be charged with the commercial ACpower. Such appliances and electronic apparatuses (hereinaftercollectively referred to electronic apparatuses) may contain a powersupply (inverter) for converting commercial AC power into DC (DirectCurrent) power. Otherwise, an inverter may be incorporated in anexternal power adapter (AC adapter) of the electronic apparatuses.

SUMMARY

The present disclosure provides various embodiments of a controlcircuit, which is capable of stably controlling a DC/DC converter basedon a voltage of an auxiliary winding.

According to one embodiment of the present disclosure, there is provideda control circuit of a DC/DC converter including a transformer having aprimary winding and an auxiliary winding provided at a primary side anda secondary winding provided at a secondary side, a switching transistorconnected to the primary winding, and a detection resistor provided on apath of the switching transistor. The control circuit includes: a pulsemodulator configured to generate a pulse signal having a duty cycleadjusted such that an output voltage of the DC/DC converter approaches atarget value, based on a detection voltage across the detection resistorand a feedback voltage depending on the output voltage of the DC/DCconverter; and a driver configured to switch the switching transistorbased on the pulse signal. The pulse modulator includes: an off signalgenerator to generate an off signal asserted depending on the feedbackvoltage and the detection voltage; and an on signal generator asserteddepending on a voltage of one end of the auxiliary winding, wherein thepulse modulator is configured to generate the pulse signal which isshifted to an on level corresponding to turning-on of the switchingtransistor when the on signal is asserted and is shifted to an off levelcorresponding to turning-off of the switching transistor when the offsignal is asserted. The on signal generator includes: a bottom detectioncomparator configured to compare the voltage of one end of the auxiliarywinding with a predetermined threshold voltage and generate a bottomdetection signal asserted when the voltage of the one end of theauxiliary winding becomes lower than the threshold voltage; a firsttime-out circuit configured to generate a first time-out signal assertedwhen the bottom detection signal is not asserted during a first time-outperiod; a second time-out circuit configured to generate a secondtime-out signal asserted when the bottom detection signal is notasserted during a predetermined second time-out period shorter than thefirst time-out period; and a logic part configured to generate the onsignal based on the bottom detection signal, the first time-out signaland the second time-out signal.

With this configuration, the switching transistor can be switched basedon the bottom detection signal if the voltage of the one end of theauxiliary winding is vibrated with no attenuation. The switchingtransistor can be switched based on the second time-out signal if thevoltage of the one end of the auxiliary winding is vibrated withattenuation. The switching transistor can be switched based on the firsttime-out signal if the voltage of the one end of the auxiliary windingdoes not have any variation due to a failure or the like. Thus, thecontrol circuit with this configuration can operate the DC/DC converterstably based on the voltage of the auxiliary winding.

The logic part may include a bottom determination unit which determineswhether or not the bottom detection signal is asserted under a statewhere the switching transistor is turned off. The logic part may beconfigured to validate the second time-out signal when it is determinedthat the bottom detection signal is asserted, and the second time-outsignal is invalidated when it is determined that the bottom detectionsignal is not asserted. With this configuration, it can be determined bythe bottom determination unit whether the voltage of the one end of theauxiliary winding is vibrated or remains at a constant value due to afailure or the like.

The bottom determination unit may generate a bottom determination signalwhich is negated when the switching transistor is turned off, andthereafter is asserted when the bottom detection signal is asserted.When the bottom detection signal is not asserted, the bottomdetermination signal continues to be negated.

The bottom determination unit may include a D flip-flop having an inputterminal to which a high level voltage is input, a clock terminal towhich the bottom detection signal is input, and a reset terminal towhich a signal to direct turning-on/off of the switching transistor isinput.

The on signal generator may further include a set mask signal generatorconfigured to generate a set mask signal asserted after lapse of apredetermined set mask time after the switching transistor is turnedoff. The logic part may assert the on signal based on one of the bottomdetection signal, the first time-out signal and the second time-outsignal, which is asserted earliest after the set mask signal isasserted.

The on signal generator may further include a bottom count controllerconfigured to generate a set signal asserted when the number of times bywhich the bottom detection signal is asserted reaches a predeterminedvalue. The logic part may assert the on signal based on the earliestasserted one of the set signal, the first time-out signal and the secondtime-out signal.

The on signal generator may further include a blanking circuitconfigured to mask the bottom detection signal during a predeterminedmask period after the switching transistor is turned off.

The off signal generator may include an error comparator configured tocompare the feedback signal depending on the output voltage of the DC/DCconverter with the detection voltage across the detection resistor andgenerate an off signal asserted based on a result of the comparison.

According to another embodiment of the present disclosure, there isprovided a control circuit including an on signal generator. The onsignal generator includes: a bottom detection comparator configured tocompare the voltage of the one end of the auxiliary winding with apredetermined threshold voltage and generate a bottom detection signalasserted when the voltage of the one end of the auxiliary windingbecomes lower than the threshold voltage; and a logic part configured toassert the on signal (i) when the bottom detection signal is assertedafter lapse of certain set mask time after the switching transistor isturned off, (ii) when a predetermined first time-out period elapsesafter the switching transistor is turned off, or (iii) when a statewhere the bottom detection signal is not asserted lasts for a secondtime-out period shorter than the first time-out period after the bottomdetection signal is asserted.

According to another embodiment of the present disclosure, there isprovided a control circuit including an on signal generator. The onsignal generator includes: a bottom detection comparator configured tocompare the voltage of the one end of the auxiliary winding with apredetermined threshold voltage and generate a bottom detection signalasserted when the voltage of the one end of the auxiliary windingbecomes lower than the threshold voltage; and a logic part configured toassert the on signal (i) when the bottom detection signal is asserted bythe predetermined number of times, (ii) when a predetermined firsttime-out period elapses after the switching transistor is turned off, or(iii) when a state where the bottom detection signal is not assertedlasts for a second time-out period shorter than the first time-outperiod after the bottom detection signal is asserted.

With this configuration, the switching transistor can be switched basedon the bottom detection signal if the voltage of the one end of theauxiliary winding is vibrated with no attenuation. The switchingtransistor can be switched based on the second time-out signal if thevoltage of the one end of the auxiliary winding is vibrated withattenuation. The switching transistor can be switched based on the firsttime-out signal if the voltage of the one end of the auxiliary windingdoes not have any variation due to a failure or the like. Thus, thecontrol circuit with this configuration can operate the DC/DC converterstably based on the voltage of the auxiliary winding.

The control circuit may be integrated on a single semiconductorsubstrate. The term “integration” may include a case where all circuitelements are formed on the single semiconductor substrate, a case wheresome main circuit elements are integrated on the single semiconductor,and a case where some resistors, capacitors and so on are formed out ofthe semiconductor substrate. When the control circuit is integrated intoa single IC (Integrated Circuit), a circuit area can be reduced andcharacteristics of circuit elements can be uniformly maintained.

According to another embodiment of the present disclosure, there isprovided a DC/DC converter including: a transformer having a primarywinding and an auxiliary winding provided at a primary side and asecondary winding provided at a secondary side; a switching transistorconnected to the primary winding of the transformer; a detectionresistor provided on a path of the switching transistor; a first diodehaving an anode connected to the secondary winding; a first outputcapacitor having one grounded end and the other end connected to acathode of the first diode; a second diode having an anode connected tothe auxiliary winding; a second output capacitor having one grounded endand the other end connected to a cathode of the second diode; a feedbackcircuit configured to generate a feedback voltage depending on an outputvoltage produced in the first output capacitor; and the above-describedcontrol circuit configured to switch the switching transistor based onthe detection voltage across the detection resistor, the voltage of theone end of the auxiliary winding and the feedback voltage.

The feedback circuit may include: a shunt regulator configured togenerate a feedback signal having a level regulated such that adifference between a voltage obtained by dividing the output voltage anda predetermined target value is zeroed; and a photo coupler having aprimary side light emitting device controlled by the feedback signal,wherein a signal produced by a secondary side light emitting device ofthe photo coupler is supplied, as the feedback signal, to the controlcircuit.

According to another embodiment of the present disclosure, there isprovided a power supply including: a filter configured to filter acommercial AC voltage; a diode rectifier circuit configured to fullwave-rectify an output voltage of the filter; a smoothing capacitorconfigured to generate a DC input voltage by smoothing an output voltageof the diode rectifier circuit; and the above-described DC/DC converterconfigured to drop the DC input voltage and supply the dropped voltageto a load.

According to another embodiment of the present disclosure, there isprovided an electronic apparatus including: a load; a filter configuredto filter a commercial AC voltage; a diode rectifier circuit configuredto full wave-rectify an output voltage of the filter; a smoothingcapacitor configured to generate a DC input voltage by smoothing anoutput voltage of the diode rectifier circuit; and the above-describedDC/DC converter configured to drop the DC input voltage and supply thedropped voltage to the load.

According to another embodiment of the present disclosure, there isprovided a power adapter including: a filter configured to filter acommercial AC voltage; a diode rectifier circuit configured to fullwave-rectify an output voltage of the filter; a smoothing capacitorconfigured to generate a DC input voltage by smoothing an output voltageof the diode rectifier circuit; and the above-described DC/DC converterconfigured to drop the DC input voltage and generate a DC outputvoltage.

Other aspects of the present disclosure may include any combinations ofthe above-described elements or conversion of expression of the presentdisclosure between methods, apparatuses and so on.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing an inverter including a DC/DCconverter.

FIG. 2 is a waveform diagram showing an operation of a control circuitof FIG. 1.

FIG. 3 is a circuit diagram showing a configuration of an on signalgenerator of a control circuit according to the embodiment.

FIG. 4 is waveform diagram showing an operation of the control circuitaccording to the embodiment.

FIG. 5 is a view showing an AC adapter including the inverter accordingto the embodiment.

FIGS. 6A and 6B are views showing an electronic apparatus equipped withthe inverter according to the embodiment.

FIG. 7 is a circuit diagram showing a configuration of an on signalgenerator according to a first modification.

DETAILED DESCRIPTION

Various embodiments of the present disclosure will now be described indetail with reference to the drawings. Throughout the drawings, the sameor similar elements, members and processes are denoted by the samereference numerals and explanation of which will not be repeated. Thedisclosed embodiments are provided for the purpose of illustration, notlimitation, of the present disclosure and all features and combinationsthereof described in the embodiments cannot be necessarily construed todescribe the spirit of the present disclosure.

In the specification, the phrase “connection of a member A and a memberB” is intended to include direct physical connection of the member A andthe member B as well as indirect connection thereof via other member aslong as the other member has no substantial effect on the electricalconnection of member A and member B or has no damage to functions andeffects shown by a combination of member A and member B. Similarly, thephrase “interposition of a member C between a member A and a member B”is intended to include direct connection of member A and member C ordirect connection of member B and member C as well as indirectconnection thereof via other member as long as the other member has nosubstantial effect on the electrical connection of the member A, themember B and the member C or has no damage to functions and effectsshown by a combination of the member A, the member B and the member C.

FIG. 1 is a circuit diagram showing an inverter 1 including a DC/DCconverter 10. The inverter 1 includes a fuse 2, an input capacitor Ci, afilter 4, a diode rectifier circuit 6, a smoothing capacitor Cs and aDC/DC converter 10.

A commercial AC voltage V_(AC) is input to the filter 4 via the fuse 2and the input capacitor Ci. The filter 4 removes a noise of thecommercial AC voltage V_(AC). The diode rectifier circuit 6 is a diodebridge circuit for full-wave rectifying the commercial AC voltageV_(AC). An output voltage of the diode rectifier circuit 6 is smoothedby the smoothing capacitor Cs and is then converted into a DC voltageVH.

The DC/DC converter 10 receives and steps down the DC voltage VH andsupplies an output voltage VOUT stabilized to a target value to a load(not shown) connected to an output terminal P2.

The DC/DC converter 10 includes a control circuit 100, an output circuit200 and a feedback circuit 210. The output circuit 200 includes atransformer T1, a first diode D1, a first output capacitor CM, aswitching transistor M1 and a detection resistor RS. The topology of theoutput circuit 200 is typical and therefore, explanation thereof will beomitted.

An output terminal (OUT terminal) of the control circuit 100 isconnected to a gate electrode of the switching transistor M1 via aresistor Rg. As the control circuit 100 switches the switchingtransistor M1, the input voltage VH is stepped down and the outputvoltage VOUT is generated. In addition, by adjusting a duty cycle of aswitching operation of the switching transistor M1, the control circuit100 controls coil current ILp flowing into a primary winding W1 of thetransformer T1 while stabilizing the output voltage VOUT to the targetvalue.

The detection resistor RS is connected in series to the primary windingW1 of the transformer T1 and the switching transistor M1. A voltage drop(detection voltage) Vcs proportional to the current ILp flowing into theprimary winding W1 and the switching transistor M1 is produced in thedetection resistor RS. The detection voltage Vcs is input to a currentdetection terminal (CS terminal) of the control circuit 100. The controlcircuit 100 controls the current ILp flowing into the primary winding W1on the basis of the detection voltage Vcs.

The feedback circuit 210 generates a feedback voltage Vfb depending onthe output voltage VOUT and supplies it to a feedback terminal (FBterminal) of the control circuit 100. The feedback circuit 210 includesa shunt regulator 212 and a photo coupler 214. The shunt regulator 212is an error amplifier, which generates a feedback signal S11 having alevel regulated such that an error between the output voltage VOUT and apredetermined target value becomes zero, and supplies the generatedfeedback signal S11 to a light emitting diode of the photo coupler 214.A photo transistor of the photo coupler 214 converts a light signal S12emitted from the light emitting diode into the feedback voltage Vfbdepending on the feedback signal S11.

The primary side of the transformer T1 has an auxiliary winding W3 inaddition to the primary winding W1. The auxiliary winding W3, a seconddiode D2 and a second output capacitor Cvcc form a second DC/DCconverter. In response to the switching of the switching transistor M1,a DC voltage Vcc is produced in the second output capacitor Cvcc. The DCvoltage Vcc is supplied to a power terminal VCC (VCC terminal) of thecontrol circuit 100. A start resistor Rstart is interposed between theVCC terminal and the input terminal P1. When starting the operation ofthe control circuit 100, the capacitor Cvcc is charged via the startresistor Rstart and the power voltage Vcc is supplied to the controlcircuit 100.

The control circuit 100 is a pulse modulator of a so-called peak currentmode and includes an edge blanking circuit 102, a pulse modulator 110and a driving circuit 130.

The detection voltage Vcs jumps temporarily after the switchingtransistor M1 is turned on. In order to prevent the switching transistorM1 from being turned off due to the jumping of the detection voltageVcs, the edge blanking circuit 102 masks the detection voltage Vcsduring a mask period immediately after the turning-on of the switchingtransistor M1.

A capacitor Cfb is externally attached to the FB terminal. The FBterminal is pulled up by a resistor R11. The feedback voltage Vfb isdivided by resistors R12 and R13.

The pulse modulator 110 generates a pulse signal S_(PM) having a dutycycle adjusted depending on the feedback voltage Vfb. The pulsemodulator 110 controls a timing at which the switching transistor M1 isturned off, based on the detection voltage Vcs proportional to the coilcurrent ILp flowing into the switching transistor M1. The drivingcircuit 130 switches the switching transistor M1 based on the pulsesignal S_(PM).

The pulse modulator 110 of FIG. 1 is a peak current mode modulator andincludes an error comparator 112, a logic part 116 and an on signalgenerator 118. The error comparator 112 compares a divided feedbackvoltage Vfb′ with a detection voltage Vcs′ and generates an off signalS_(OFF) asserted when the detection voltage Vcs′ reaches the dividedfeedback voltage Vfb′.

The control circuit 100 has an auxiliary terminal (ZT terminal). Avoltage Va of one end of the auxiliary winding W3 is divided byresistors Rzt1 and Rzt2 (not shown). A divided voltage (or a ZT voltage)Vzt is input to the ZT terminal.

The on signal generator 118 generates an on signal S_(ON) asserted whenthe voltage of one end of the auxiliary winding W3 becomes lower than apredetermined threshold voltage Vth_zt.

Energy is stored in the transformer T1 during an on period of theswitching transistor M1, whereas energy is released during an off periodof the switching transistor M1. The voltage Va remains at a certainlevel of voltage in a period during which current ILs flows into asecondary winding W2 immediately after the switching transistor M1 isturned off. When the energy of the transformer T1 is zeroed and thecurrent ILs is zeroed, the voltage Va is vibrated by a quasi-resonance.When the voltage Va is reduced to near zero by the vibration, the onsignal generator 118 determines that the energy of the transformer T1 iszeroed, and then asserts the on signal S_(ON) to turn on the switchingtransistor M1 again.

The logic part 116 is a SR flip-flop and has a set terminal S to whichthe on signal S_(ON) is input and a reset terminal R to which the offsignal S_(OFF) is input. The output (or a pulse modulated signal) S_(PM)of the logic part 116 is shifted to an on level (high level)corresponding to turning-on of the switching transistor M1 whenever theon signal S_(ON) is asserted, and is shifted to an off level (low level)corresponding to the turning-off of the switching transistor M1 wheneverthe off signal S_(OFF) is asserted.

The driving circuit 130 switches the switching transistor M1 based onthe pulse signal S_(PM).

FIG. 2 is a waveform diagram showing an operation of the control circuit100 of FIG. 1. At time t1, the off signal S_(OFF) is asserted and theswitching transistor M1 is turned off. While energy remains in thetransformer T1 after the switching transistor M1 is turned off, the ZTvoltage Vzt of the ZT terminal is maintained at a certain level. Whenthe energy of the transformer T1 is zeroed, a voltage across the primarywinding W1 and a voltage across the auxiliary winding W3 begin to bevibrated by a resonance circuit that includes the primary winding W1 anda capacitor Cds (at time t2).

The on signal generator 118 compares the ZT voltage Vzt with thepredetermined threshold voltage Vth_zt and generates a bottom detectionsignal BOTTOM_DET asserted whenever Vzt<Vth_zt. A hysteresis is set forthe threshold voltage Vth_zt.

Set mask time τ_(SETMASK) is measured to set the upper limit of aswitching frequency of the switching transistor M1. A set mask signalSET_MASK is asserted after lapse of the set mask time τ_(SETMASK) afterthe switching transistor M1 is turned off (at time t3). The bottomdetection signal BOTTOM_DET in the set mask time τ_(SETMASK) is masked(invalidated).

When the bottom detection signal BOTTOM_DET is asserted after lapse ofthe set mask time τ_(SETMASK), the on signal generator 118 r asserts theon signal S_(ON) (at time t4).

When the on signal S_(ON) is asserted, the pulse signal S_(PM) isshifted to an on level and the switching transistor M1 is turned on.When the switching transistor M1 is turned on, the coil current ILpincreases with time and the detection voltage Vcs′ rises accordingly.When the detection voltage Vcs′ reaches the feedback voltage Vfb′, theoff signal S_(OFF) is asserted and the switching transistor M1 is turnedoff again (at time t5).

The control circuit 100 repeats the above operation.

In the DC/DC converter 10 of FIG. 1, there is a possibility that the ZTvoltage Vzt is attenuated while being vibrated after time t2. The ZTvoltage Vzt at this time is indicated by an alternate long and shortdash line in FIG. 2. If the ZT voltage Vzt remains lower than thethreshold voltage Vth_zt, the on signal S_(ON) cannot be asserted andthe switching transistor M1 cannot be accordingly turned on again.

In addition, if the resistor Rzt2 is circuit-shorted or the outputterminal of the DC/DC converter 10 is grounded, the ZT voltage Vzt willbe at near 0V and the on signal S_(ON) cannot be accordingly asserted.

The DC/DC converter 10 according to an embodiment of the presentdisclosure includes the control circuit 100, the output circuit 200 andthe feedback circuit 210.

The control circuit 100 according to the embodiment may be a functionalIC integrated on a single semiconductor substrate. The control circuit100 includes the pulse modulator 110, the driving circuit 130 and theedge blanking circuit 102. The pulse modulator 110 is a peak currentmode modulator and generates a pulse signal S_(PM) having a duty cycleadjusted such that an output voltage VOUT of the DC/DC converter 10approaches a target value, based on a voltage Va of one end of anauxiliary winding W3, a detection voltage Vcs across a detectionresistor RS, and a feedback voltage Vfb depending on the output voltageVOUT of the DC/DC converter 10.

The driving circuit 130 switches the switching transistor M1 based onthe pulse signal S_(PM). The driving circuit 130 includes a pre-driver132 and a driver 134.

The pulse modulator 110 includes the error comparator (off signalgenerator) 112, the on signal generator 118 and the logic part 116.

The error comparator 112 generates an off signal S_(OFF) asserteddepending on the feedback voltage Vfb and the detection voltage Vcs.Specifically, the error comparator 112 compares a feedback voltage Vfb′with a detection voltage Vcs′ and generates the off signal SOFF asserted(having high level) when Vfb′<Vcs′.

The on signal generator 118 generates an on signal S_(ON) asserteddepending on a voltage Va of one end of the auxiliary winding W3.

The logic part 116 is a SR flip-flop and has a set terminal S to whichthe on signal S_(ON) is input and a reset terminal R to which the offsignal S_(OFF) is input. An output signal (or a pulse modulated signal)S_(PM) of the logic part 116 is shifted to an on level (high level)corresponding to turning-on of the switching transistor M1 whenever theon signal S_(ON) is asserted, and is shifted to an off level (low level)corresponding to turning-off of the switching transistor M1 whenever theoff signal S_(OFF) is asserted. The logic part 116 may be configured asa logic device other than an SR flip-flop.

FIG. 3 is a circuit diagram showing a configuration of the on signalgenerator 118 of the control circuit 100 according to the embodiment.The on signal generator 118 includes a logic part 401, a bottomdetection comparator 402, a ZT blanking circuit 403 and a one-shotcircuit 408.

The bottom detection comparator 402 compares a ZT voltage Vzt with apredetermined threshold voltage Vth_zt and generates a bottom detectionsignal S31 asserted when the ZT voltage Vzt becomes lower than thethreshold voltage Vth_zt.

There is a possibility that the ZT voltage Vzt notably swingsimmediately after the switching transistor M1 is turned off, and abottom is incorrectly detected despite the fact that energy remains inthe transformer T1. The ZT blanking circuit 403 is provided to preventsuch incorrect bottom detection. The ZT blanking circuit 403 masks thebottom detection signal S31 during a certain masking period after theswitching transistor M1 is turned off.

A blanking timer 406 generates a ZT mask signal S32 having a high levelduring a certain masking period after a gate signal NOUT of an N channelMOSFET of the driver 134 is shifted to a high level. An AND gate 404masks the bottom detection signal S31 by performing an AND operation forthe bottom detection signal S31 and the ZT mask signal S32.

A bottom detection signal S33 output from the ZT blanking circuit 403 isasserted whenever the ZT voltage Vzt decreases to near the bottom afterthe switching transistor M1 is turned off.

The one-shot circuit 408 generates a bottom detection signal S34 havinga high level during a certain period after the bottom detection signalS33 is asserted.

A first time-out circuit 412 a generates a first time-out signal S35asserted when a state where the bottom detection signal S33 is negatedlasts for a first time-out period τ1 (for example, 15 μs). The firsttime-out circuit 412 a is reset whenever the bottom detection signal S33is asserted, and counts up while the bottom detection signal S33 isbeing negated. When a count value reaches a value corresponding to thefirst time-out period τ1, the first time-out signal S35 is asserted.

A second time-out circuit 412 b generates a second time-out signal S36asserted when the state where the bottom detection signal S33 is negatedlasts for a second time-out period τ2 (for example, 5 μs) shorter thanthe first time-out period τ1. The second time-out circuit 412 b is resetwhenever the bottom detection signal S33 is asserted, and counts upwhile the bottom detection signal S33 is being negated. When a countvalue reaches a value corresponding to the second time-out period τ2,the second time-out signal S36 is asserted.

The logic part 401 generates the on signal S_(ON) based on the bottomdetection signal S34, the first time-out signal S35 and the secondtime-out signal S36.

The logic part 401 includes a bottom determining unit 440 to determinewhether or not the bottom detection signal S33 is asserted under a statewhere the switching transistor M1 is turned off, in other words, whetheror not the ZT voltage Vzt is vibrated.

The bottom determining unit 440 is negated when the switching transistorM1 is turned off, and then generates a bottom determination signal S37asserted when the bottom detection signal S34 is asserted. For example,the bottom determining unit 440 includes a D flip-flop 442 having aninput terminal to which a high level voltage is input, a clock terminalto which the bottom detection signal S34 is input, and a reset terminal(having inverted logic) to which a signal #NOUT to direct turning-on/offof the switching transistor M1 is input. The signal #NOUT is a signalproduced by inverting the signal NOUT by an inverter 418. Whenever thesignal NOUT is shifted to a high level, in other words, whenever theswitching transistor M1 is turned off, the D flip-flop 442 is reset andthe bottom determination signal S37 has a low level (i.e., is negated).When the bottom detection signal S34 is asserted, the bottomdetermination signal S37 has a high level (i.e., is asserted).

An AND gate 444 generates a bottom determination signal S38 byperforming an AND operation for the bottom determination signal S37 andthe bottom detection signal S33.

An AND gate 446 masks the second time-out signal S36 with the bottomdetermination signal S37. Thus, the second time-out signal S36 isvalidated when it is determined that the bottom detection signal S34 isasserted, whereas the second time-out signal S36 is invalidated when itis determined that the bottom detection signal S34 is not asserted.

A set mask signal generator 430 generates a set mask signal S41 asserted(having a high level) after lapse of certain set mask time τ_(SETMASK)after the switching transistor M1 is turned off.

The logic part 401 asserts the on signal S_(ON) based on one of thebottom detection signal S34, the first time-out signal S35 and thesecond time-out signal S36, whichever is asserted earliest after the setmask signal S41 is asserted. Specifically, an OR gate 414 generates alogical sum of the bottom detection signal S34, the first time-outsignal S35 and the second time-out signal S36. This corresponds toselecting the earliest asserted one of these three signals. An AND gate416 generates the on signal S_(ON) by performing an AND operation of anoutput S40 of the OR gate 414 and the set mask signal S41.

In other words, the logic part 401 asserts the on signal S_(ON) (i) whenthe bottom detection signal S34 is asserted after lapse of a certain setmask time τ_(SETMASK) after the switching transistor M1 is turned off,(ii) when the first time-out period τ1 elapses after the switchingtransistor M1 is turned off, or (iii) when a state where the bottomdetection signal S34 is not asserted lasts for the second time-outperiod τ2 after the bottom detection signal S34 is asserted.

Subsequently, an operation of the control circuit 100 will be described.FIG. 4 is a time chart showing an operation of the control circuit 100.

A time period between time t0 and time t1 shows an operation whenstarting the control circuit 100. At time t0, the control circuit 100 isstarted. Immediately after the starting point, the ZT voltage Vztremains at a ground voltage. The second time-out signal S36 is assertedafter lapse of the second time-out period τ2 from the starting point,and the first time-out signal S35 is asserted after lapse of the firsttime-out period τ1. Since the bottom detection signal S34 is notasserted immediately after the starting point, the bottom determinationsignal S38 has a low level and the second time-out signal S36 isinvalidated. At time t1 after the lapse of the first time-out period τ1,when the first time-out signal S35 is asserted, the on signal S_(ON) isasserted and the switching transistor M1 is turned on.

When the switching transistor M1 is turned on, current ILp of theprimary winding W1 increases to raise the detection voltage Vcs′. Whenthe detection voltage Vcs′ reaches the feedback voltage Vfb′, the offsignal S_(OFF) is asserted, a switching signal S_(OUT) has a low leveland the switching transistor M1 is turned off again (at time t2).

After time t2, the ZT voltage Vzt is vibrated while being attenuated ina turning-off period of the switching transistor M1. It is shown in FIG.4 that the bottom detection signal S34 has a low level after beingasserted three times. With the lapse of the second time-out period τ2after the bottom detection signal S34 is asserted at the third time, thesecond time-out signal S36 is asserted. Since the bottom determinationsignal S37 is asserted by the bottom detection signal S34 at the firsttime, the second time-out signal S36 is validated and the switchingtransistor M1 is turned on by the second time-out signal S36.

After time t3, failure such as grounding of the output terminal of theDC/DC converter 10 and short-circuit of the resistor Rzt2 may occur. Inthis case, after the switching transistor M1 is turned off, the ZTvoltage Vzt remains at the ground voltage (0V) and the bottom detectionsignal S34 is not asserted. After the switching transistor M1 is turnedoff, the second time-out signal S36 is asserted after lapse of thesecond time-out period τ2 and the first time-out signal S35 is assertedafter lapse of the first time-out period τ1. After the switchingtransistor M1 is turned off, since the bottom detection signal S34 isnot asserted even once, the bottom determination signal S37 is negatedand, accordingly, the second time-out signal S36 is invalidated. As aresult, the switching transistor M1 is turned on based on the firsttime-out signal S35.

According to the control circuit 100, if the voltage Va of the one endof the auxiliary winding W3, i.e., the ZT voltage Vzt, is vibratedwithout being attenuated, the switching transistor M1 is switcheddepending on the bottom detection signal S34.

If the ZT voltage Vzt is vibrated with attenuation, the switchingtransistor M1 is switched depending on the second time-out signal S36.

When the ZT voltage Vzt does not have any variation due to a failure orthe like, the switching transistor M1 is switched depending on the firsttime-out signal S35.

Accordingly, the control circuit 100 allows the DC/DC converter 1 to bestably operated based on the voltage Va of the auxiliary winding W3depending on conditions of the DC/DC converter 1.

Advantages of the control circuit 100 can become more apparent from acomparison with a comparative technique employing only one time-outcircuit. In the comparative technique, if a time-out period is set to belong (for example, 15 μs), there is a large difference between aswitching frequency at which the ZT voltage Vzt is vibrated withattenuation and a switching frequency at which the ZT voltage Vzt isvibrated with no attenuation, which may result in output voltagevariation and sound noise due to frequency variation.

On the other hand, if the time-out period is set to be short (forexample, 5 μs), there is a small difference between the switchingfrequency in the attenuated vibration and the switching frequency in thenon-attenuated vibration, which may result in reduction in outputvoltage variation and sound noise. However, this is not preferable sincea high switching frequency needs to be maintained even when the ZTvoltage Vzt is maintained at near the ground voltage due to a failure.

However, the control circuit 100 according to the embodiment canovercome the above problems of the comparative technique since twotime-out periods are set and the switching frequency is automaticallyand optimally controlled depending on conditions of the DC/DC converter10.

An application of the DC/DC converter 10 will be described next. Theinverter 1 including the DC/DC converter 10 is suitable to be used foran AC adapter or a power block of an electronic apparatus.

FIG. 5 is a view showing an AC adapter 800 including the inverter 1. TheAC adapter 800 includes a plug 802, a housing 804 and a connector 806.The plug 802 receives a commercial AC voltage V_(AC) from an electricalsocket (not shown). The inverter 1 is mounted within the housing 804. ADC output voltage VOUT generated by the inverter 1 is supplied from theconnector 806 to an electronic apparatus 810. Examples of the electronicapparatus 810 may include a notebook PC, a digital camera, a digitalvideo camera, a mobile phone, a portable audio player and the like.

FIGS. 6A and 6B are views showing an electronic apparatus 900 includingthe inverter 1. Although it is illustrated with a display apparatus inFIGS. 6A and 6B, the electronic apparatus 900 is not particularlylimited but may be any apparatus incorporating a power supply, such asan audio apparatus, a refrigerator, a washing machine, a vacuum cleanerand the like. A plug 902 receives a commercial AC voltage V_(AC) from anelectrical socket (not shown). The inverter 1 is mounted within ahousing 904. A DC output voltage VOUT generated by the inverter 1 issupplied to a load mounted within the same housing 904, such as amicrocomputer, a DSP (Digital Signal Processor), a power supply circuit,an illumination apparatus, an analog circuit, a digital circuit or thelike.

Heretofore, the present disclosure has been described by way of specificembodiments. The disclosed embodiments are merely examples and it is tobe understood by those skilled in the art that combinations of elementsand processes of the embodiments can be modified in various ways andsuch modification falls within the scope of the present disclosure. Thefollowing description is given to such modification.

(First Modification)

FIG. 7 is a circuit diagram showing a configuration of an on signalgenerator 118 a according to a first modification. The on signalgenerator 118 a includes a bottom count controller 410 as a substitutionfor the set mask signal generator 430 of the on signal generator 118 ofFIG. 3.

The bottom count controller 410 generates a set signal S42 asserted whenthe number of times by which the bottom detection signal S34 is assertedreaches a predetermined value. The OR gate 414 generates the on signalS_(ON) by performing an OR operation for the set signal S42, the firsttime-out signal S35 and the second time-out signal S36. That is, thelogic part 401 asserts the on signal S_(ON) based on the earliestasserted one of the set signal S42, the first time-out signal S35 andthe second time-out signal S36.

In this modification, the on signal generator 118 a asserts the onsignal S_(ON) (i) when the bottom detection signal S34 is asserted bythe predetermined number of times, (ii) when a certain first time-outperiod τ1 elapses after the switching transistor M1 is turned off, or(iii) when a state where the bottom detection signal S34 is not assertedlasts for the second time-out period τ2 after the bottom detectionsignal S34 is asserted.

The first modification can obtain the same effects as the aboveembodiments.

(Second Modification)

Although it has been illustrated in the above embodiments that the shuntregulator (error amplifier) 212 is disposed at the secondary side of thetransformer T1, the error amplifier may be disposed at the primary sidethereof or may be incorporated into the control circuit 100.

Although it has been illustrated in the above embodiments that thecircuits described are constructed by the positive logic (high-active)system in which assertion of each signal is assigned with a high leveland negation thereof is assigned with a low level, the circuits may beconstructed by a negative logic system or a combination of the positivelogic system and the negative logic system.

According to the present disclosure in some embodiments, it is possibleto provide a control circuit which is capable of stably controlling aDC/DC converter based on a voltage of an auxiliary winding.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the disclosures. Indeed, the novel methods and apparatusesdescribed herein may be embodied in a variety of other forms;furthermore, various omissions, substitutions and changes in the form ofthe embodiments described herein may be made without departing from thespirit of the disclosures. The accompanying claims and their equivalentsare intended to cover such forms or modifications as would fall withinthe scope and spirit of the disclosures.

What is claimed is:
 1. A control circuit of a DC/DC converter includinga transformer having a primary winding and an auxiliary winding providedat a primary side and a secondary winding provided at a secondary side,a switching transistor connected to the primary winding, and a detectionresistor provided on a path of the switching transistor, the controlcircuit comprising: a pulse modulator configured to generate a pulsesignal having a duty cycle adjusted such that an output voltage of theDC/DC converter approaches a target value, based on a detection voltageacross the detection resistor and a feedback voltage depending on theoutput voltage of the DC/DC converter; and a driver configured to switchthe switching transistor based on the pulse signal, wherein the pulsemodulator includes: an off signal generator configured to generate anoff signal asserted depending on the feedback voltage and the detectionvoltage; and an on signal generator asserted depending on a voltage ofone end of the auxiliary winding, wherein the pulse modulator isconfigured to generate the pulse signal which is shifted to an on levelcorresponding to turning-on of the switching transistor when the onsignal is asserted and is shifted to an off level corresponding toturning-off of the switching transistor when the off signal is asserted,and wherein the on signal generator includes: a bottom detectioncomparator configured to compare the voltage of the one end of theauxiliary winding with a predetermined threshold voltage and generate abottom detection signal asserted when the voltage of the one end of theauxiliary winding becomes lower than the threshold voltage; a firsttime-out circuit configured to generate a first time-out signal assertedwhen the bottom detection signal is not asserted during a first time-outperiod; a second time-out circuit configured to generate a secondtime-out signal asserted when the bottom detection signal is notasserted during a predetermined second time-out period shorter than thefirst time-out period; and a logic part configured to generate the onsignal based on the bottom detection signal, the first time-out signaland the second time-out signal.
 2. The control circuit of claim 1,wherein the logic part includes a bottom determination unit whichdetermines whether or not the bottom detection signal is asserted undera state where the switching transistor is turned off, and wherein thelogic part is configured to validate the second time-out signal when itis determined that the bottom detection signal is asserted, andinvalidate the second time-out signal when it is determined that thebottom detection signal is not asserted.
 3. The control circuit of claim2, wherein the bottom determination unit generates a bottomdetermination signal which is negated when the switching transistor isturned off, and thereafter is asserted when the bottom detection signalis asserted.
 4. The control circuit of claim 3, wherein the bottomdetermination unit includes a D flip-flop having an input terminal towhich a high level voltage is input, a clock terminal to which thebottom detection signal is input, and a reset terminal to which a signalto direct turning-on/off of the switching transistor is input.
 5. Thecontrol circuit of claim 1, further comprising a set mask signalgenerator configured to generate a set mask signal asserted after lapseof a predetermined set mask time after the switching transistor isturned off, wherein the logic part asserts the on signal based on one ofthe bottom detection signal, the first time-out signal and the secondtime-out signal, which is asserted earliest after the set mask signal isasserted.
 6. The control circuit of claim 1, wherein the on signalgenerator further includes a bottom count controller configured togenerate a set signal asserted when the number of times by which thebottom detection signal is asserted reaches a predetermined value,wherein the logic part asserts the on signal based on the earliestasserted one of the set signal, the first time-out signal and the secondtime-out signal.
 7. The control circuit of claim 1, wherein the onsignal generator further includes a blanking circuit configured to maskthe bottom detection signal during a predetermined mask period after theswitching transistor is turned off.
 8. The control circuit of claim 1,wherein the off signal generator includes an error comparator configuredto compare the feedback signal depending on the output voltage of theDC/DC converter with the detection voltage across the detection resistorand generate an off signal asserted based on a result of the comparison.9. The control circuit of claim 1, wherein the control circuit isintegrated on a single semiconductor substrate.
 10. A DC/DC convertercomprising: a transformer having a primary winding and an auxiliarywinding provided at a primary side and a secondary winding provided at asecondary side; a switching transistor connected to the primary windingof the transformer; a detection resistor provided on a path of theswitching transistor; a first diode having an anode connected to thesecondary winding; a first output capacitor having one end connected toa ground and the other end connected to a cathode of the first diode; asecond diode having an anode connected to the auxiliary winding; asecond output capacitor having one end connected to the ground and theother end connected to a cathode of the second diode; a feedback circuitconfigured to generate a feedback voltage depending on an output voltageproduced in the first output capacitor; and a control circuit of claim1, wherein the control circuit is configured to switch the switchingtransistor based on the detection voltage across the detection resistor,the voltage of the one end of the auxiliary winding and the feedbackvoltage.
 11. An electronic apparatus comprising: a load; a filterconfigured to filter a commercial AC voltage; a diode rectifier circuitconfigured to full wave-rectify an output voltage of the filter; asmoothing capacitor configured to generate a DC input voltage bysmoothing an output voltage of the diode rectifier circuit; and a DC/DCconverter of claim 10, which is configured to drop the DC input voltageand supply the dropped voltage to the load.
 12. A power adaptercomprising: a filter configured to filter a commercial AC voltage; adiode rectifier circuit configured to full wave-rectify an outputvoltage of the filter; a smoothing capacitor configured to generate a DCinput voltage by smoothing an output voltage of the diode rectifiercircuit; and a DC/DC converter of claim 10, which is configured to dropthe DC input voltage and generate a DC output voltage.
 13. The DC/DCconverter of claim 10, wherein the feedback circuit includes: a shuntregulator configured to generate a feedback signal having a levelregulated such that a difference between a voltage obtained by dividingthe output voltage and a predetermined target value is zeroed; and aphoto coupler having a primary side light emitting device controlled bythe feedback signal, wherein a signal produced by a secondary side lightemitting device of the photo coupler is supplied, as the feedbacksignal, to the control circuit.
 14. A power supply comprising: a filterconfigured to filter a commercial AC voltage; a diode rectifier circuitconfigured to full wave-rectify an output voltage of the filter; asmoothing capacitor configured to generate a DC input voltage bysmoothing an output voltage of the diode rectifier circuit; and a DC/DCconverter of claim 10, which is configured to drop the DC input voltageand supply the dropped voltage to a load.
 15. A control circuit of aDC/DC converter including a transformer having a primary winding and anauxiliary winding provided at a primary side and a secondary windingprovided at a secondary side, a switching transistor connected to theprimary winding, and a detection resistor provided on a path of theswitching transistor, the control circuit comprising: a pulse modulatorconfigured to generate a pulse signal having a duty cycle adjusted suchthat an output voltage of the DC/DC converter approaches a target value,based on a detection voltage across the detection resistor and afeedback voltage depending on the output voltage of the DC/DC converter;and a driver configured to switch the switching transistor based on thepulse signal, wherein the pulse modulator includes: an off signalgenerator configured to generate an off signal asserted depending on thefeedback voltage and the detection voltage; and an on signal generatorasserted depending on a voltage of one end of the auxiliary winding,wherein the pulse modulator is configured to generate the pulse signalwhich is shifted to an on level corresponding to turning-on of theswitching transistor when the on signal is asserted and is shifted to anoff level corresponding to turning-off of the switching transistor whenthe off signal is asserted, and wherein the on signal generatorincludes: a bottom detection comparator configured to compare thevoltage of the one end of the auxiliary winding with a predeterminedthreshold voltage and generate a bottom detection signal asserted whenthe voltage of the one end of the auxiliary winding becomes lower thanthe threshold voltage; and a logic part configured to assert the onsignal (i) when the bottom detection signal is asserted after lapse ofcertain set mask time after the switching transistor is turned off, (ii)when a predetermined first time-out period elapses after the switchingtransistor is turned off, or (iii) when a state where the bottomdetection signal is not asserted lasts for a second time-out periodshorter than the first time-out period after the bottom detection signalis asserted.
 16. A control circuit of a DC/DC converter including atransformer having a primary winding and an auxiliary winding providedat a primary side and a secondary winding provided at a secondary side,a switching transistor connected to the primary winding, and a detectionresistor provided on a path of the switching transistor, the controlcircuit comprising: a pulse modulator configured to generate a pulsesignal having a duty cycle adjusted such that an output voltage of theDC/DC converter approaches a target value, based on a detection voltageacross the detection resistor and a feedback voltage depending on theoutput voltage of the DC/DC converter; and a driver configured to switchthe switching transistor based on the pulse signal, wherein the pulsemodulator includes: an off signal generator configured to generate anoff signal asserted depending on the feedback voltage and the detectionvoltage; and an on signal generator asserted depending on a voltage ofone end of the auxiliary winding, wherein the pulse modulator isconfigured to generate the pulse signal which is shifted to an on levelcorresponding to turning-on of the switching transistor when the onsignal is asserted and is shifted to an off level corresponding toturning-off of the switching transistor when the off signal is asserted,and wherein the on signal generator includes: a bottom detectioncomparator configured to compare the voltage of the one end of theauxiliary winding with a predetermined threshold voltage and generate abottom detection signal asserted when the voltage of the one end of theauxiliary winding becomes lower than the threshold voltage; and a logicpart configured to assert the on signal (i) when the bottom detectionsignal is asserted by the predetermined number of times, (ii) when apredetermined first time-out period elapses after the switchingtransistor is turned off, or (iii) when a state where the bottomdetection signal is not asserted lasts for a second time-out periodshorter than the first time-out period after the bottom detection signalis asserted.
 17. A control method of a DC/DC converter including atransformer having a primary winding and an auxiliary winding providedat a primary side and a secondary winding provided at a secondary side,a switching transistor connected to the primary winding, and a detectionresistor provided on a path of the switching transistor, the controlmethod comprising: comparing a voltage of one end of the auxiliarywinding with a predetermined threshold voltage and generating a bottomdetection signal asserted when the voltage of the one end of theauxiliary winding becomes lower than the threshold voltage; asserting anon signal (i) when the bottom detection signal is asserted after lapseof certain set mask time after the switching transistor is turned off,(ii) when a predetermined first time-out period elapses after theswitching transistor is turned off, or (iii) when a state where thebottom detection signal is not asserted lasts for a second time-outperiod shorter than the first time-out period after the bottom detectionsignal is asserted; generating an off signal asserted based on adetection voltage across the detection resistor and a feedback voltagedepending on an output voltage of the DC/DC converter; generating apulse signal which is shifted to an on level corresponding to turning-onof the switching transistor when the on signal is asserted and isshifted to an off level corresponding to turning-off of the switchingtransistor when the off signal is asserted; and driving the switchingtransistor based on the pulse signal.
 18. A control method of a DC/DCconverter including a transformer having a primary winding and anauxiliary winding provided at a primary side and a secondary windingprovided at a secondary side, a switching transistor connected to theprimary winding, and a detection resistor provided on a path of theswitching transistor, the control method comprising: comparing a voltageof one end of the auxiliary winding with a predetermined thresholdvoltage and generating a bottom detection signal asserted when thevoltage of the one end of the auxiliary winding becomes lower than thethreshold voltage; asserting an on signal (i) when the bottom detectionsignal is asserted by the predetermined number of times, (ii) when apredetermined first time-out period elapses after the switchingtransistor is turned off, or (iii) when a state where the bottomdetection signal is not asserted lasts for a second time-out periodshorter than the first time-out period after the bottom detection signalis asserted; generating an off signal asserted based on a detectionvoltage across the detection resistor and a feedback voltage dependingon an output voltage of the DC/DC converter; generating a pulse signalwhich is shifted to an on level corresponding to turning-on of theswitching transistor when the on signal is asserted and is shifted to anoff level corresponding to turning-off of the switching transistor whenthe off signal is asserted; and driving the switching transistor basedon the pulse signal.